1. Field of the Invention:
The present invention relates to a rotor, a manufacturing method thereof and an electric gear motor having the rotor.
2. Description of Related Art
A previously known rotatable worm shaft, which forms a rotatable shaft of an electric gear motor, is provided with a bearing, such as a ball bearing, that is fixed to the worm shaft at a location between a worm portion of the worm shaft, which is formed at a distal end side and has a worm, and a base end side portion of the worm shaft, at which a rotor core is fixed.
For instance, in a case of the motor recited in Japanese Unexamined Patent Publication No. 2009-195027A, at the time of fixing the ball bearing to the rotatable shaft, an annular stop ring, which is configured into a C-shape, is installed into an annular groove of the shaft. Then, the ball bearing is received over the shaft. Thereafter, a cylindrical fixing member is received over the rotatable shaft from the opposite axial side of the ball bearing, which is opposite from the stop ring, so that the ball bearing is clamped between the stop ring and the cylindrical fixing member and is thereby axially securely positioned.
In another case of the motor recited in Japanese Unexamined Patent Publication No. 2009-207297A, a first stop ring, which is configured into a C-shape, is installed into a first annular groove of the shaft. Thereafter, the ball bearing is received over the shaft. Then, a second stop ring, which is configured into a C-shape, is installed into a second annular groove of the shaft, which is provided on an opposite axial side of the ball bearing that is opposite from the first annular groove. Furthermore, another annular processing groove is formed in the shaft on an axial side of the second annular groove, which is opposite from the first annular groove, so that a radial projection is bulged, i.e., is produced between the second annular groove and the processing groove. Then, this bulged radial projection and the second stop ring are axially pressed against the ball bearing, so that the ball bearing is axially securely positioned relative to the shaft. Furthermore, in Japanese Unexamined Patent Publication No. 2009-207297A, instead of forming the processing groove, it is also recited to form an annular projection in the shaft at a location, which is on an axial side of the first groove and is adjacent to the first groove. Then, the radial projection and the first stop ring are axially pressed against the ball bearing, so that the ball bearing is axially securely positioned relative to the shaft.
Furthermore, in another case of the motor recited in Japanese Unexamined Utility Model Publication No. H06-9359U, the ball bearing, which is received over the shaft, is axially clamped between a stop ring, which is configured into a C-shape and is installed into an annular groove of the shaft, and a housing of the motor. In this way, the ball bearing is securely positioned relative to the rotatable shaft.
Furthermore, in another case recited in Japanese Unexamined Utility Model Publication No. H06-70461U or Japanese Examined Utility Model Publication No. H06-9563Y, an annular projection is bulged, i.e., is produced in the outer peripheral surface of the shaft by pressing the outer peripheral surface of the shaft, and the ball bearing, which is received over the shaft, is clamped between the bulged annular projection and the housing of the motor. In this way, the ball bearing is securely positioned relative to the rotatable shaft.
However, in the case of fixing the ball bearing to the shaft in the manner recited in Japanese Unexamined Patent Publication No. 2009-195027A, Japanese Unexamined Patent Publication No. 2009-207297A or Japanese Unexamined Utility Model Publication No. H06-9359U, the groove forming process for forming the groove on the shaft should be performed, and the separate component(s), such as the stop ring or fixing member, should be installed to the shaft. Therefore, the processing costs, the component costs and the assembling costs are disadvantageously increased.
Furthermore, when the rotatable shaft, which has the annular projection, is formed like in the case of Japanese Unexamined Patent Publication No. 2009-207297A, a shaft preform having an outer diameter, which is equal to the outer diameter of the annular projection, may be processed through a cutting process to form such a shaft. In such a case, the shaft preform, which has the large diameter, i.e., the shaft preform, which is relatively expensive in terms of its material costs due to the increased amount of the material, is used to form the shaft. A majority of the shaft preform is cut to form the annular projection in the shaft, so that the large amount of waste material is generated. Therefore, the material costs and the processing costs become expensive, and the amount of waste material becomes large.
Furthermore, when the ball bearing is fixed to the shaft in the manner discussed in Japanese Unexamined Utility Model Publication No. H06-70461U or Japanese Examined Utility Model Publication No. H06-9563Y, the ball bearing cannot be held in place relative to the shaft until the rotor is assembled to the housing. Therefore, the ball bearing may possibly be detached from the shaft in the middle of the assembling. In order to alleviate the above disadvantages, it is conceivable to form the annular projection (swaged projection) in the outer peripheral surface of the shaft at two opposed axial sides, respectively, of the ball bearing. In such a case, the ball bearing is inserted over the shaft until it contacts the first one of the annular projections (swaged projections). Thereafter, the second one of the annular projections is formed in the outer peripheral surface of the shaft on the opposite axial side of the ball bearing, which is opposite from the first one of the annular projections. However, at the time of forming the second one of the annular projections, an axial force is applied to the ball bearing, so that the first one of the annular projections may be deformed to cause an axial positional deviation of the ball bearing. Therefore, it is difficult to accurately position the ball bearing relative to the shaft. Furthermore, the ball bearing is inserted over the shaft between the formation of the first one of the annular projections and the second one of the annular projections. Therefore, the time required to fix the ball bearing is disadvantageously lengthened.